1. Field of the Invention
The present invention relates to a gas sensor control apparatus which also detects the H2O gas concentration of a gas under measurement by using a gas sensor which detects the concentration of a specific gas contained in the gas under measurement.
2. Description of the Related Art
Examples of conventionally known gas sensors for detecting the concentration of a specific gas contained in a gas under measurement, such as exhaust gas discharged from an internal combustion engine, include an oxygen sensor for detecting the concentration of oxygen and an NOx sensor for detecting the concentration of nitrogen oxide (NOx). These gas sensors include a sensor element(s) composed of a solid electrolyte body mainly made of zirconia. For example, a full-range air-fuel ratio sensor whose output changes linearly with oxygen concentration has two sensor elements; i.e., an electromotive force cell and a pump cell. The current flowing between the electrodes of the pump cell is controlled such that the voltage generated between the electrodes of the electromotive force cell becomes constant, and the oxygen concentration is detected from the magnitude of the current flowing through the pump cell.
Patent Document 1 discloses a gas concentration/humidity detection apparatus which detects not only the concentration of a specific gas contained in a gas under measurement, but also the humidity of the gas under measurement. In this gas sensor, the detected concentration of the specific gas (e.g., oxygen concentration) is corrected on the basis of the detected humidity.
In the gas concentration/humidity detection apparatus disclosed in Patent Document 1, when the humidity of the gas under measurement (namely, the concentration of H2O gas contained in the gas under measurement) is detected, the control target voltage of the electromotive force cell is switched from a first reference voltage (e.g., 450 mV) at which the H2O gas contained in the gas under measurement does not substantially dissociate, to a second reference voltage (e.g., 1000 mV) at which the H2O gas contained in the gas under measurement dissociates. The H2O gas concentration is detected on the basis of first and second currents which are pump currents detected when the first and second reference voltages are used, respectively.    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No 2010-281732